1. Field of the Invention
This invention relates to a light deflector for scanning an original image or a photosensitive member with a beam of light irradiated from a laser light source, for example, in an image reading system of a digital copying machine, a facsimile or a like apparatus or an image writing system of a laser beam printer or a like apparatus.
2. Description of the Related Art
Conventionally, in a reading system for an original image, a method of irradiating a beam of light upon an original image to scan the original image to obtain density information for individual picture elements from reflected light is known, and in a writing system for a recording image, another method of irradiating a beam of light, which is modulated in accordance with picture information, upon a photosensitive member to scan the photosensitive member to form an electrostatic latent image is known. In both systems, an optical scanning system which employs a light deflector including a polygon mirror is known as a system for scanning an original image or a photosensitive member with a beam of light irradiated from a laser light source.
FIG. 8 is a schematic view showing an image writing system which employs such scanning system. Referring to FIG. 8, the image writing system shown includes a laser light source 100, a collimator lens 101, a light deflector 102 including a polygon mirror 102a, an f-.theta. lens 103, and a photosensitive drum 104. A beam of light irradiated from the laser light source 100 is reflected by a reflecting mirror face of the polygon mirror 102a and introduced to the photosensitive drum 104. Thereupon, the beam of light is deflected by the polygon mirror 102a as the polygon mirror 102a rotates in the direction indicated by an arrow mark A so that it scans the photosensitive drum 104 along the direction indicated by an arrow mark B. Meanwhile, the photosensitive drum 104 rotates in the direction indicated by an arrow mark C, and a two-dimensional electrostatic latent image is formed on the photosensitive drum 104.
Conventionally, a light deflector which is used for such an application as described above is disclosed in Japanese Patent Laid-Open Application No. Sho 59-23324 and Japanese Utility Model Laid-Open Application No. Hei 3-81915. In particular, the light deflector is constructed such that a rotary member is supported for rotation on a fixed shaft mounted uprightly on a housing while a polygon mirror is secured to a mirror flange formed on the rotary member and is rotated together with the rotary member by a motor section incorporated in the housing. Further, in the conventional light deflector, the polygon mirror is pressed against the mirror flange by a mirror cap screwed on an outer periphery of the rotary member and is thus held and fixed between and by the mirror cap and the mirror flange.
In the conventional light deflector, however, since the mirror cap is screwed directly on the rotary member, if the mirror cap is fastened tightly to the rotary member in order to fix the polygon mirror with security, then the rotary member is compressed toward the inner side, resulting in the following problem. In particular, in light deflectors in recent years, in order to satisfy the demands for an increase of the speed of rotation and a decrease of vibrations of a polygon mirror and so forth, a dynamic pressure pneumatic bearing is employed to support the rotary member in a contactless condition on the fixed shaft. In this instance, since the gap (hereinafter referred to as bearing clearance) between the fixed shaft and the rotary member is only approximately 3 mm, if the rotary member is compressed toward the inner side, then the fixed shaft and the rotary member are brought into contact with each other, and consequently, rotation of the polygon mirror is disturbed. Or, even if the fixed shaft and the rotary member are not contacted with each other, if the bearing clearance varies, then rotation of the rotary member becomes non-uniform, resulting in non-uniform rotation of the polygon mirror.